An electrocardiograph (ECG) system monitors heart activity in a human patient. The ECG system applies small conductive pads or "electrodes" to specific locations on the human patient. Each electrode has an adhesive patch and a conductive member (which might be one in the same) that is placed on the patient's skin. These electrodes detect electrical impulses generated by the heart during each beat. Each heart beat generates a waveform that consists of three identifiable wave complexes referred to as the P, QRS, and T wave complexes.
To appropriately capture the heart beat waveform, a conventional 12-lead resting ECG is conducted using ten electrodes. An electrode is adhered to each of the four limbs of the human patient--left arm, right arm, left leg, and right leg--and to six anatomically-prescribed locations across the chest and left side of the human patient.
In response to detection of the electrical impulses from the heart, the electrodes produce electrical signals indicative of the heart activity. These electrical signals have small magnitudes on the order of 1 mV, and are commonly resolved by signal processes down to about 5 .mu.V. The electrical signals are different from one another by virtue of the different physical locations of the electrodes about the patient. FIG. 1 shows a diagrammatic representation of a conventional ECG system 10 having ten electrodes V1-V6, RA, LA, LL, and RL connected to a human patient 12. The electrodes are connected to an ECG device 14 via conductors 16. ECG device 14 detects the electrical signals generated by the electrodes and performs various signal processing and computational operations to convert the raw electrical signals into meaningful information that can be displayed or printed out for review by a physician.
From the signals produced by the ten electrodes, ECG device 14 can produce two sets of ECG leads: limb leads and chest leads. The "limb leads" are formed from the right arm electrode RA, the left arm electrode LA, and the left leg electrode LL as follows:
I=LA-RA PA1 II=LL-RA PA1 III=LL-LA PA1 aVR=RA-1/2(LA+LL) PA1 aVL=LA-1/2(RA+LL) PA1 aVF=LL-1/2(LA+RA) PA1 Lead V1=V1-(RA+LA+LL)/3 PA1 Lead V2=V2-(RA+LA+LL)/3 PA1 Lead V3=V3-(RA+LA+LL)/3 PA1 Lead V4=V4-(RA+LA+LL)/3 PA1 Lead V5=V5-(RA+LA+LL)/3 PA1 Lead V6=V6-(RA+LA+LL)/3.
In addition to these six leads, six "chest leads" are formed by subtracting the average of the right arm, left arm, and left leg electrode outputs from each of the chest electrodes V1-V6, as follows:
Notice that each of these twelve ECG leads of a conventional 12-lead ECG test concerns input from only three of the four limb electrodes: the right arm electrode RA, the left arm electrode LA, and the left leg electrode LL. According to medical convention, the right leg electrode RL is not used in deriving a multi-channel ECG recording. Part of this is due to the RL electrode being located farthest from the heart in comparison to the other nine electrodes.
The right leg electrode RL is used, however, to help reduce common-mode interference that appears between the signal leads and a common reference plane. As illustrated in FIG. 1, it is common for the patient to have an electric potential V.sub.P-E relative to a true electrical earth ground, while the ECG device has a relative or floating ground ECG GND that is at a different electric potential V.sub.E-ECG relative to true earth ground. Typically, ECG device 14 is configured so that its own ground ECG GND tries to approximate true earth ground, thereby leaving a difference in potential between the patient 12 and ECG device 14. These different potentials cause common-mode noise voltage in the electrode signals carried by conductors 16 which appears equally and in phase from each electrode/conductor relative to ground.
Conventional ECG systems are designed to reject the common-mode signals. Because each electrode leadwire has the same common mode signal, the 12 lead tests described above are each derived by subtracting different ones of the electrical signals so that theoretically the common mode interference is removed. However, subtraction alone may not completely eliminate the common mode interference because there is such a significant difference in magnitudes between the common mode signal and the small signals being detected by the electrodes. For example, an electric potential of a human patient relative to electric earth ground might be 5-10 volts, which is one to several orders of magnitude greater that the 1 mV signal detected by the electrode and the resolvable unit of 5 .mu.V. Simply subtracting signals at the 5-10 volt range might result in missing important information at the 1 mV or less range.
Accordingly, it is conventional to apply a small correcting current to the right leg electrode RL in an effort to bring the electric potential of the human patient and the relative ground of the ECG system in line with each other. As diagrammatically shown in FIG. 1, the RL electrode is driven in such a way that the patient-to-earth ground voltage potential V.sub.P-E and the ECG device-to-earth ground potential V.sub.E-ECG are approximately equal, thereby substantially removing the common mode interference before the EGG even conducts the lead computations. Conventionally, the RL electrode is based upon the average of the other three limb electrodes RA, LL, and LA. An operational amplifier 22 provides a correcting current based upon a difference between the average voltage from these three limbs and the EGG GND.
Designers of EGG systems are also cognizant of the need to accommodate DC offset voltages which may exist between leadwires, originating from electrochemical mechanisms inherent to the skin-electrode interfaces. Performance standards (e.g., ANSI) require an accuracy in signal detection over a 300 mV range. To test compliance with these standards, a designer conducts a test where one conductor is driven to +300 mV relative to all other conductors and a reading is taken to detect electrical signals. Then that same conductor is driven to -300 mV relative to all other conductors and another reading is taken. This action is repeated for each of the conductors. The system passes if signals can be detected over the entire .+-.300 mV range.
As shown in FIG. 1, ECG device 14 typically has an amplifying subsystem 18 which amplifies the analog signals generated by the electrodes and an analog-to-digital (A/D) converter 20 which converts the amplified analog signals into digital values that are resolved down to approximately 5 .mu.V. To accommodate a 600 mV (i.e., .+-.300 mV) signal acquisition range, a 17-bit A/D converter is used to account for the 120,000 resolvable increments (i.e., 600 mV/5 .mu.V). Other techniques might alternatively be used to detect signals within the 600 mV range, such as a hardware high-pass pole.
It would be beneficial to reduce the dynamic range from 600 mV to a narrower signal acquisition range. This reduction would simplify the signal acquisition circuitry, and lower its cost, while still complying with medical requirements. For instance, a reduction by one-half to a 300 mV acquisition range would permit a designer to employ conventional, off-the-shelf 2.sup.n devices (e.g., 16-bit A/D converter) to capture the signals, rather than unconventional electronics like the illustrated 17-bit A/D converter that is used to detect signals within the 600 mV range.
It is an aspect of this invention to provide an ECG system that satisfies the requirements for offset range and resolution, as specified by ANSI and others, while operating within a reduced dynamic range for signal detection.